Planar multiwavelength optical power supply on a silicon platform

ABSTRACT

A microphotonic light source includes an optical pump and a plurality of waveguides that distribute optical pump power of the optical pump. At least one Erbium-doped laser ring is coupled to at least one of the waveguides so as to match the resonance condition of the optical pump.

PRIORITY INFORMATION

This application claims priority from provisional application Ser. No.60/491,778 filed Aug. 1, 2003, which is incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

The invention relates to the field of microphotonic light sources, andin particular to a planar multiwavelength optical power supply on asilicon platform.

Photonics technology revolutionized telecommunication systems in thelater 1970s with the deployment of low-loss, single mode silica fibersand efficient, double heterostructure, single mode injection lasers.Long haul fiber optic systems were ultimately enabled with thedevelopment of erbium-doped fiber amplifiers (EDFA's), operating aroundthe 1.55 μm low-loss fiber communication regime, dubbed the C-band. TheEDFA's gain bandwidth is optimal within a 4000 GHz (30 nm) wide spectralwindow about 1.55 μm, thus permitting the eventual integration ofwavelength division multiplexing (WDM) communications in the late 1990s.

Within the microelectronics industry, the continued drive of Moore's Lawtowards smaller circuit elements and denser chip architecture has nowyielded to an intra-chip transmission limitation referred to as theinterconnect bottleneck, an RC circuit delay occurring due to thesmaller cross-section and closer spacing of conducting metal lines abovethe integrated circuit chip.

The recent growth of Metropolitan Area Networks (MAN), boosted by therapid growth of the internet and a resulting demand for highercommunication bandwidth, has given impetus to the development ofmicron-scale integrated photonic devices (microphotonics), creating anew generation range of faster, high yielding, higher functionalitycomplex photonic devices. These micron-scale structures present aconvergence of solutions for both the interconnect bottleneck problem,and the MAN requirement of low-cost high volume components.

Si-based microphotonics is a planar waveguide technology, combining allthe necessary components for optical signal transmission and computinginto a single optical chip, on a silicon wafer platform. Thesecomponents include, lasers, switches, modulators, detectors, and channeladd/drop filters. A significant reduction in cost and system sizeresults from this dense planar integration, where all the opticalcomponents with different functionalities can be combined to yield anIntegrated Optical Platform (IOP) or Optoelectronic Integrated Circuit(OEIC's) fully compatible with the widespread CMOS siliconmicroelectronics technology. This compatibility with CMOS technologyleverages a powerful degree of planar processing experience in favor ofdesigning high performance photonic structures. Silicon IOP would allowunprecedented information and computing capabilities appealing for avariety of technological markets.

The scientific and technological breakthrough of silicon microphotonicswould consist of the demonstration of an efficient, CMOS compatiblelaser source for the silicon platform, enabling the integration ofinformation processing silicon microelectronics with high bandwithtransmitting silicon-based photonic structures. The invention proposedhere provides a multi-frequency laser platform operating in the C-bandtelecommunication window with full CMOS compatibility.

Lasers and optical amplifiers are essential components of all opticalcircuits. Since light emission from silicon is an intrinsicallyinefficient process, current methods for realizing laser devices inintegrated optical circuits involve expensive materials (mainly III-Vsemiconductor compounds) and deposition technologies (MBE, CVD) thatcannot be easily integrated within CMOS silicon processing fabricationline.

However, passive silicon microphotonics, comprised of photonicstructures performing light-guiding, routing and processing functions,has boomed during the last ten years. A near-complete operational set ofphotonics devices have been demonstrated: silicon based opticalwaveguides with extremely low losses and small curvature radii, tunableoptical filters, fast switches (ns), fast optical modulators (GHz), fastphotodetectors, integrated Ge photodetectors for 1.55 μm radiation.Micromechanical MEMS system and full band-gap photonic crystals havebeen demonstrated while switching systems are already commercial.

In the face of this passive component integration, the primarylimitation to realizing a fully autonomous IOP is an efficient activeSi-based device, such as a light emitting diode (LED) or a laser lightsource.

Silicon is an indirect band-gap material, resulting in light emissionthrough a low probability phonon-mediated process (spontaneousrecombination lifetimes in the ms range). In standard bulk silicon,competitive non-radiative recombination rates are much higher than thisradiative rate, and the majority of excited excess electron-hole pairsrecombine without photon emission, thus yielding a very low internalquantum efficiency (η_(i)≈10⁻⁶). In addition, fast non-radiativeprocesses such as Auger or free carrier absorption severely preventpopulation inversion for silicon optical transitions at the high pumpingrates needed to achieve optical amplification.

Despite of all these difficulties, during the 1990s several strategieshave been investigated to cope with the intrinsically poor lightemission yield of silicon.

Among the different approaches developed to overcome this materiallimitation, quantum confinement and rare earth doping of silicon havedominated the scientific efforts around active silicon microphotonics.

Due to the favorable modification of their optical properties, numeroussilicon nanostructures, such as porous silicon, silicon nanocrystalsembedded in an SiO₂ matrix, and Si/SiO₂ superlattices have been widelystudied. The efficient, tunable and visible room temperatureluminescence of all these structures has been ascribed to therecombination of quantum confined excitons which are self-trapped in asize dependent Si═O level at the interface between the siliconnanostructure and the SiO₂ matrix.

Rare earth doping studies of Erbium (Er) doped crystalline silicon havedemonstrated that Er can be excited in Si through electron-hole pairrecombination or through impact excitation by high energy carriers,yielding Er-doped LEDs operating at room temperature. However, a verypoor quantum efficiency and high intraband absorption rules out thepossibility of realizing a laser or optical amplifier with such aphotonic structure.

Er-doping of glass structures reveals an ideal approach since itproduces an almost temperature independent emission line originatingfrom an internal 4-f shell transition. Even though erbium doped SiO₂ isused commercially to realize optical fiber amplifiers, the applicationof erbium-based structures in silicon microphotonics is limited so farby the small optical cross section of Er³⁺ transitions, and representsone of the major challenges of silicon-based microphotonics.

Recently, erbium doping of Si nanocrystals has been recognized as ahybrid method combining the promising features of both the describedmethods. Indeed, it has been demonstrated that Si nanocrystals in thepresence of Er act as efficient sensitizers for the light emission ofthe rare earth. The effective excitation cross section of erbium ions inpresence of Si nanocrystals is more than two orders of magnitudes higherwith respect to the resonant absorption of a photon in a silica matrixwhile, in addition, non radiative de-excitation processes are stronglysuppressed.

Recent observations of net optical gain at 1.54 μm with enhanced erbiumemission cross section in Er-doped Si nanocluster sensitized waveguides,and the demonstration of efficient room temperature electro-luminescencefrom Er-silicon nanocrystal devices has opened the route towards thefuture fabrication of Si IOP devices based on Er amplification.

Room temperature continuous wavelength lasing action in the nearinfra-red and visible has been demonstrated by optical pumping ofmicrodisk structures, comprised of III-V InP based materials. Enhancedphoto-luminescence from Er-doped microdisks has been observed,establishing experimental groundwork for our realization of an Er-dopedmicrodisk laser.

In addition, the fabrication of ultra-high Q toroid Er-doped microcavityon a silicon chip and the realization of planar Er-doped silicamicrodisk structures with extremely smooth edges is paving the waytowards the realization of low noise, CMOS compatible erbium-based microlaser devices and optical amplifiers.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided amicrophotonic light source. The microphotonic light source includes anoptical pump and a plurality of waveguides that distribute optical pumppower of the optical pump. At least one Erbium-doped laser ring iscoupled to at least one of the waveguides so as to match the resonancecondition of the optical pump.

According to another aspect of the invention, there is provided a methodof forming a microphotonic light source. The method comprises providingan optical pump and providing a plurality of waveguides that distributeoptical pump power of the optical pump. Also, the method includesproviding at least one Erbium-doped laser ring that is coupled to atleast one of the waveguides so as to match the resonance condition ofthe optical pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a Broadband MicrophotonicLight Source (BMLS) design layout;

FIG. 2 is a graph demonstrating the pump intensity dependent on erbiumgain coefficient;

FIG. 3 is a graph demonstrating the gain-loss figure of the invention;

FIG. 4 is a graph demonstrating the Gain in the device versus thequality factor;

FIG. 5 is a graph demonstrating the steady state average photon number;

FIG. 6 is a graph demonstrating the bending attenuation factor forSi₃N₄:Er rings

FIG. 7 is a graph demonstrating the bending attenuation factor foroxinitride:Er rings; and

FIG. 8 is a graph demonstrating the performance of the output channels.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a Broadband Microphotonic Light Source (BMLS) that includesa multiwavelength laser source emitting within the telecommunicationsC-Band, around 1.55 μm, which is fully compatible with planar opticaltechnology. In the scheme of the device, a pump laser (tunable oroperating at fixed wavelength) distributes optical pump power throughsilicon nitride (Si₃N₄) or silicon oxynitride (SiON) waveguides 2 toErbium-doped Si₃N₄ or SiON microring laser resonators 4 with, radialdimensions designed to match the resonance condition for both the pumpand the signal wavelengths. Routing of the pump wavelength across thisoptical circuit is achieved by coupling the pump radiation of a(tunable) 980 nm laser source with undoped microrings or routing rings4.

The routing rings 4 aligned along a principal input waveguide bus 2 todistribute the input pump power to regions of the optical circuitcontaining different radii microring lasers 6. The microring lasers 6are optically pumped and lased at different wavelengths, as determinedby their ring radii, lying within the emission spectrum of the Er atom.The device scheme is easily implemented on a silicon platform, providingan integrated and comparatively inexpensive approach towards therealization of a tunable multi-wavelength C-band (1.55 μm) laser sourcefor silicon-based opto-electronic integrated circuits.

The invention schematically sketched in FIG. 1 includes of a planararray of Er-doped microring laser sources with multi-wavelength signalemission around the 1.55 μm region and with the option of a selectiveoptical pump to provide an equivalent switching and routingfunctionality. With respect to the standard Er-doped fiber ring laserapproach, the microring laser device is capable of monolithicalintegration with standard silicon technology and provides a routetowards the achievement of very large scale integration (VLSI) inoptical photonic circuits (PCs).

The invention is made of a planar multiwavelength laser source realizedby coupling of Er-doped microring resonators with passive microrings ofdifferent dimensions acting as selective pump routers and signalfilters, for a tunable 980 nm pump laser. The passive router microrings4 are all aligned along a common waveguide bus 2 for the pumptransmission while the active Er-doped microrings +6 are coupled withindividual routing waveguides, as shown in FIG. 1. The Er-dopedmicrorings are resonantly coupled with the signal erbium emissionwavelength and with the 980 nm pump in order to enhance the pumpingefficiency of the active structures through pump trapping in the activering.

The big advantage of the Er-related gain mechanism is the strategic 1.55μm emission combined with a flat gain spectrum, due to the inhomogeneousbroadening of the erbium transitions in the host material. In principle,the tunable planar laser platform can be realized out of any broad gainmaterial that can be optically excited. The Q factors of the individualerbium rings can be increased by selectively heating and reflowing theactive rings in order to create smoother edge peripheral regions in therings.

The planar active platform has great flexibility and can be designed indifferent configurations. The routing rings 4 can have different radiito allow multiple pump sources or can be all the same when the platformis intended as a selective wavelength switcher or a single pump source.

Even if the router rings 4 are resonant only at the pump wavelength,additional filtering devices 8, 10 can be needed to avoid signalback-coupling with these rings. Additional low Q filtering (Q≅500) rings10 with resonant wavelengths λ_(m) and λ_(m+1) can be used to avoid thisproblem (or additional Bragg grating with high reflectance at the signalwavelength and high transmission for the pump can be utilized ifnecessary).

A pump recycling strategy, additional bus waveguides or drop ports 8,can be employed to get rid of back coupled pump and signal arising fromthe strong overcoupling regime. The platform can also be realizeddirectly on a two dimensional hexagonal full photonic band gapstructures (such as air holes in the SiO₂ overcladding) with a band-gapcenter at the signal frequency, in order to suppress stray signals frompropagating into the plane of the platform and thus enhance the lasermicroring's Q factor (by inhibiting ring radiation loss).

Active SiON rings can be enriched by nano-silicon inclusions (throughstandard sputtering procedures) or silicon nanocrystals in order to takeadvantage from the energy transfer mechanism to erbium ions and use anenhanced erbium emission cross section to yield considerably higher gainvalues in the device. The pump laser can be either 980 nm or 1480 nm.

The BMLS is a single planar photonic structure that emits multiplewavelengths of laser light within the telecommunication C-band. Thesemultiple wavelengths emerge from output planar waveguides 12, where onewaveguide 12 is dedicated to each generated wavelength of laser light,as shown in FIG. 1. The value of such a multi-wavelength coherent lightsource is in its immediate applicability to WDM communications:integrated with external absorption modulators, this device is amulti-channel Gbit/s output transmitter, powered by a single (or atunable) 980 nm optical pump.

For laser microrings designed with an amorphous structure, such asdeposited Si₃N₄ or SiON, the Er atom's luminescence and gain emissionspans a 100 nm range. FIG. 9 shows the room temperaturephoto-luminescence profile of Er in SiO₂, a similar such glassstructure, for reference. The gain bandwidth is approximated as aspectral window comprising the majority of this luminescence, anddefined to be roughly 1470-1570 nm for the Si₃N₄ and SiON based BMLSdesign.

The three laser microrings 6 radii are chosen to resonantly trap boththe optical pump wavelength (980 nm) and a lasing wavelengths that liewithin this window (see Table 1). Er is a long ms-lifetime emitter, as aresult of which total device quality factors Q in excess of 100 000 arerequired in order to achieve lasing within the laser microring, as shownin FIGS. 2 and 5. This implies the requirement of a significantly lowradiative and transmission loss coefficient, below 0.1 dB/cm. Thedimensions of our ring structure, as shown in Table 1, imply a minimumradius of 8 μm for design of any ring structure; in the case of thelaser microring, a ring radii near 20 μm is chosen, resulting in a freespectral range (FSR) of 6.5-7 nm. For the particular case of the lasermicroring with radius 21.1 μm, this FSR implies the existence of ninehigh intensity lasing wavelengths within the 1470-1570 nm gainbandwidth, as shown in FIG. 8.

These nine high intensity lasing wavelengths propagate within the lasermicroring in clockwise and counter-clockwise direction, and couple tothe routing waveguide in a forwards and backwards propagating direction.The backwards propagating laser wavelengths do not resonantly couple tothe routing ring, and are guided to output on the side of the BMLSopposite to that for the forwards propagating laser wavelengths.

For both the forward and backward propagating laser wavelengths, filterrings selectively pick off each of the laser wavelengths from therouting waveguides, dropping each laser wavelength into on outputwaveguide. The smaller diameter of the filter rings (see example offilter ring with radius 9.25 μm in FIG. 1) implies a larger freespectral range (15 nm, for this particular case), which enables thefilter ring to selectively pick up only one laser wavelength from therouting wavelength and drop it into the output waveguide, and not pickoff any of the other eight laser wavelengths, as shown in FIG. 8. Onelaser microring thus generates an output of 18 light sources, made up of9 unique laser wavelengths. With three laser microrings within ourdesign, the BMLS emits a total of 54 laser sources, made up of 27 uniquelaser wavelengths, all within the telecommunication C-band.

While the Q of the laser microring must be in excess of 100 000 in orderto ensure lasing action, the Q of the router microring and filtermicroring can be much lower, on the order of 400 to 500.

An alternative design to enhance coupling of pump power into the lasermicroring is detailed in FIG. 1 for one of the laser microringschematics: placing a second waveguide next to the laser microringtheoretically predicts the possibility of more efficient coupling of thesiphoned optical pump power into this microring. This design doubles thenumber of output waveguides emitting laser light and allows a simplescheme for pump photon recycling.

The BMLS is a planar structure that will be patterned in onelithographic step. The process flow requires wet thermal oxidation of 4inch or 6 inch silicon Si wafers with 3 μm or 6 μm of silicon oxide SiO₂on both sides of the wafer. This layer of SiO₂ acts as an opticalisolator, isolating the propagating modes of our photonic structuresfrom being evanescently coupled into the Si wafer substrate.

The micron-scale layer of Si₃N₄ co-doped with Er, or SiON co-doped withEr, or Si-rich Si₃N₄ co-doped with Er, or Si-rich SiON co-doped with Eris deposited by reactive magnetron sputtering or a combination of CVDdeposition and ion implantation. The layer of Er-doped material (with anEr doping concentration of 2-4×10²⁰ cm⁻³) is now lithographicallypatterned by conventional UV light source lithography (a positivephoto-resist is spun onto the layer of film and the resist is exposed toUV light while under an I-line stepper mask) to design the architectureof the BMLS, as schematically shown in FIG. 1. The resulting pattern isetched into the deposited layer by reactive ion etching, and theoverlying exposed photo-resist pattern is removed by plasma etching.

The resulting patterned structure is now placed under a PECVD depositionof SiO₂, of thickness 3 μm, and this is termed the over cladding layerof SiO₂. A rapid thermal anneal or longer time anneal may be required atthis step, in order to optimally activate luminescence from the Eratoms, or to form an adequate number of Si-nanocrystal sensitizers.

Er is a chemical element compliant with CMOS standards of lifetimecontamination for MOS devices. As such, this entire fabrication processflow is compliant with a CMOS-level clean room fabrication facility.

A major concern related to the use of erbium-doper SiON rings as activematerial system stems from the very low erbium emission cross section,which requires a preliminary careful design of the device structure inorder to obtain the conditions for laser oscillations. Based on a simplerate equation modeling one may determine how the relevant laserparameters, such as the gain coefficient and the threshold laser qualityfactor Q, depend on the 980 nm pump intensity, demonstrating thefeasibility of the proposed active device.

To obtain laser oscillation one need to fulfill the gain clampingcondition:g_(th)=σΔN_(th)=α  (1)where σ is the erbium emission cross section, ΔN_(th) is the populationinversion density required to yield a steady state gain coefficientg_(th) equal to the total (coupling or reflection+scattering androughness) optical losses α.

Losses are translated into a characteristic photon escaping time τthrough the device group velocity ν_(g):τ⁻¹=αν_(g)  (2)and substituting (2) into (1) one can express the laser thresholdpopulation inversion as:

$\begin{matrix}{{\Delta\; N_{th}} = \frac{1}{\sigma\;\tau\; v_{g}}} & (3)\end{matrix}$Using the general quality factor definition:

$\begin{matrix}{Q = \frac{\omega_{0}\tau}{2}} & (4)\end{matrix}$where ω₀ is the device operation pulsation related to the opticalfrequency f=ω₀/2π. Based on the previous relations it is easy to expressthe threshold quality factor to the threshold inversion density:

$\begin{matrix}{Q_{th} = \frac{\omega_{0}}{2\;\sigma\;\Delta\; N_{th}v_{g}}} & (5)\end{matrix}$

If one relates the threshold inversion level ΔN_(th) with the requiredpumping rate to build up such an inversion in the system, a veryinteresting relation expressing Q_(th) versus the pumping conditionsrequired to get laser oscillation is found.

Within a standard three level recombination scheme the steady statepopulation inversion can be generally expressed as:

$\begin{matrix}{{\Delta\; N} = {N_{0}\frac{\left( {{\sigma_{P}\phi_{P}} - w} \right)}{\left( {{\sigma_{P}\phi_{P}} + w + {2{nB}}} \right)}}} & (6)\end{matrix}$where n is the number of emitted photons, B the Einstein stimulated(emission/absorption) coefficient, σ_(P) is the absorption cross sectionat the pump wavelength, φ_(P) is the pump photon flux, and w is therecombination rate of the erbium excited level (⁴I_(13/2)). Equation (6)can be easily obtained solving in the steady state the following rateequation system:

$\begin{matrix}{{\frac{\mathbb{d}N_{2}}{\mathbb{d}t} = {{\sigma_{P}\phi_{P}N_{1}} - {wN}_{2} - {{nB}\left( {N_{2} - N_{1}} \right)}}}{N_{0} = {N_{1} + N_{2}}}} & (7)\end{matrix}$where N_(i) are the atomic level population densities. Considering thatat the laser threshold the number of photons emitted by stimulatedemission is negligible one can set n≈0 and substitute equation (6) into(5) assuming ΔN_(th)=ΔN(n=0) to obtain the pump dependent laser qualityfactor Q at threshold:

$\begin{matrix}{Q_{th} = {\frac{\omega_{0}}{2\;\sigma\; v_{g}}\frac{{\sigma_{P}\phi_{P}} + w}{N_{0}\left( {{\sigma_{P}\phi_{P}} - w} \right)}}} & (8)\end{matrix}$where φ_(p) is the pump flux required to obtain the laser thresholdpopulation inversion needed to satisfy the gain clamping relation (1). Aplot of relation (8) is shown in FIG. 2. The laser quality factor Q isgenerally related to the optical loss α in the system through:

$\begin{matrix}{Q = {\frac{\omega_{0}\tau}{2} = {\frac{\omega_{0}}{2}\frac{1}{\alpha\; v_{g}}}}} & (9)\end{matrix}$In a ring resonant system the pass gain G is given by:G=gl_(eff)  (10)where g is the gain per unit length and l_(eff) is an effectiveinteraction length proportional to the quality factor Q through:

$\begin{matrix}{l_{eff} = {{\tau\; v_{g}} = \frac{2\;{Qv}_{g}}{\omega_{0}}}} & (11)\end{matrix}$

It will be appreciated that G is a linear function, as shown in FIG. 4,of the quality factor given a constant gain per unit length (determinedby imposing the gain clamping condition at the operation pump power).

If the general expression g_(th)=σΔN_(th) is substituted for thethreshold gain into equation (10), the obvious result obtained is:G_(th)=g_(th)l_(eff)=1  (12)showing that the single pass threshold gain G is independent of Q andis 1. This fact is just the expression in different words of the gainclamping condition g_(th)=1/l_(eff)=1/τν_(g)=α and represents a soundconsistency check of our formalism.

In FIG. 2, the pump dependent gain coefficient obtained by solving themodel rate equations at the steady state is shown. The model parametervalues are reported in the FIG. 3. A maximum gain of about 2 dB/cm canbe obtained at the erbium concentration considered in the model.

In FIG. 3, the threshold quality factor Q_(th) is plotted versus pumpintensity at 980 nm. The horizontal lines represent the loss Q factorscorresponding to the optical loss (through relation 9) values indicatedin the FIG. 3. In the description, laser oscillations will start at theintersection values of the horizontal lines with the red curverepresenting the Q_(th) versus pump intensity.

On the right axis of FIG. 3 is shown the calculated inversion fractionin the system corresponding to the pump intensity at threshold. Ingeneral, to satisfy the laser oscillations condition g_(th)≧α thehorizontal lines corresponding to the loss values α have to stay on thetop part of the graph with respect to the interception point with theQ_(th) (related to the gain) red curve.

From this analysis it is clear that the minimum Q factor to get laseroscillation in the device has to be of the order of 10⁵ if the opticallosses are as high as 0.4 dB/cm. Such Q factor values are easilyachievable with our processing facilities.

FIG. 4 is a plot of the Gain in the device versus the quality factor.Gain values as high as 100 dB can be achieved for Q factors around 10⁶.

A major issue to be considered here in order to demonstrate thefeasibility of laser action in the proposed device is that of the ruleof bending losses.

Bending loss increases exponentially when the bending radius (ringradius) R decreases, and when the bend loss starts to increase it risesso sharply that a small change in the bend radius can have a dramaticeffect on the overall loss. This reflects in the need for very accuratefabrication techniques. Here numerical estimations are performed of theoptical losses and critical bending radius using the standardperturbative approach, where the zero-th order input parameters of themodel comes from an equivalent straight and symmetric slab waveguide ofthickness h.

The following bend-loss formula for the slab can be also applied to atwo-dimensional waveguide by employing the effective index method toobtain an equivalent slab structure.

The bending loss attenuation coefficient per unit length α is given by:

$\begin{matrix}{{\alpha = \frac{\gamma\;\kappa^{2}{\mathbb{e}}^{h\;\gamma}{\mathbb{e}}^{- U}}{\left( {n_{core}^{2} - n_{cl}^{2}} \right)k_{0}^{2}{\beta\left( {{h/2} + \gamma^{- 1}} \right)}}}\text{where:}} & (13) \\{U = {\left\lbrack {{\frac{\beta}{\gamma}{\ln\left( \frac{1 + {\gamma\;\beta^{- 1}}}{1 - {\gamma\;\beta^{- 1}}} \right)}} - 2} \right\rbrack\gamma\; R}} & (14)\end{matrix}$where the parameters γ and κ are given respectively by:γ=√{square root over (β² −k ₀ ² n ² _(cl))}  (15)κ=√{square root over (k ₀ ² n ² _(core)−β²)}  (16)

The bending loss estimations are shown in FIGS. 6 and 7 for the case ofa Si₃N₄ and oxinitride Er-doper rings respectively.

The steady state average photon number in the ring can be calculatedeasily starting from the basic cavity rate equation:

$\begin{matrix}{\frac{\mathbb{d}n}{\mathbb{d}t} = {{v_{g}\sigma\frac{l}{L}\left( {N_{2} - N_{1}} \right)\left( {n + 1} \right)} - {\frac{l}{L}v_{g}g_{th}n}}} & (17)\end{matrix}$Equation (17) can be solved at steady state to yield:

$\begin{matrix}{\overset{\_}{n} = \frac{\sigma\left( {N_{2} - N_{1}} \right)}{g_{th} - {\sigma\left( {N_{2} - N_{1}} \right)}}} & (18)\end{matrix}$The general expression for population inversion in a three level systemis:

$\begin{matrix}{{\Delta\; N} = \frac{\left( {R - w} \right)N_{0}}{R + w + {2{\sigma\phi}}}} & (19)\end{matrix}$where R=σ_(P)φ_(P) is the pumping rate and φ is the cavity signal photonflux.Using the general definition of saturation photon number given by:

$\begin{matrix}{n_{sat} = {\frac{P + w}{2v_{g}\sigma}V}} & (20)\end{matrix}$where V is the cavity volume, and remembering that φ=nν_(g)/V one cancast the equation (19) in a simple form:

$\begin{matrix}{{\Delta\; N} = \frac{\left( {P - w} \right)N_{0}}{\left( {P + w} \right)\left( {1 + \frac{\overset{\_}{n}}{n_{sat}}} \right)}} & (21)\end{matrix}$

Substituting g_(th)=σΔN_(th) into equation (18) and defining the newvariables

$x = {{\frac{N_{2} - N_{1}}{\Delta\; N_{th}}\mspace{14mu}{and}\mspace{14mu} y} = {\frac{\Delta\; N_{0}}{\Delta\; N_{th}}\mspace{14mu}\left( {{{where}\mspace{14mu}\Delta\; N_{0}} = \frac{\left( {P - w} \right)N_{0}}{\left( {P + w} \right)}} \right.}}$is the small signal population difference), equations (18) and (21) canbe written in the simpler form:

$\begin{matrix}{{\overset{\_}{n} = \frac{x}{1 - x}}{and}} & (22) \\{x = \frac{y}{1 + \frac{\overset{\_}{n}}{n_{sat}}}} & (23)\end{matrix}$Equations (22) and (23) can be solved simultaneously to find n.

The following quadratic expression can be derived:n ² +n _(sat)(1−y) n−n _(sat) y=0  (24)which has the solution:

$\begin{matrix}{\frac{\overset{\_}{n}}{n_{sat}} = {{\frac{1}{2}\left( {y - 1} \right)} + {\frac{1}{2}\sqrt{\left( {y - 1} \right)^{2} + \frac{4y}{n_{sat}}}}}} & (25)\end{matrix}$that represents the general solution of the problem.

The average steady state photon number can be translated into a lightintensity according to the relation

$I = {{hv}\mspace{14mu}\frac{v_{g}}{V}\mspace{14mu}{\overset{\_}{n}.}}$

The simulation results corresponding to different values of the totaloptical losses (yielding a corresponding total quality factor Q_(T)) areshown in FIG. 5. A laser threshold of about 300 μW can be extracted fora ring structure with a quality factor in excess of 10⁵.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A microphotonic light source comprising: an optical pump; a pluralityof waveguides that distribute optical pump power of said optical pump,said waveguides comprising silicon nitride (Si₃N₄) or silicon oxynitride(SiON) waveguides; a plurality of Erbium-doped laser ring structuresthat are coupled to at least one of said waveguides, each of saidErbium-doped laser rings having radial dimensions designed to match theresonance condition for both said optical pump and associatedwavelengths, said Erbium-doped laser ring structures comprisingErbium-doped Si₃N₄ or SiON and having multi-wavelength signal emissionsat approximately 1.55 μm; and a plurality of undoped routing ringstructures aligned along a principal waveguide bus to distribute thepower from the optical pump to regions comprising said Erbium-dopedlaser ring structures; wherein said Erbium-doped laser ring structuresand said undoped routing ring structures are of varying dimensions andact as selective pump routers and signal filters.
 2. The microphotoniclight source of claim 1, wherein said waveguides comprise siliconnitride Si₃N₄.
 3. The microphotonic light source of claim 1, whereinsaid waveguides comprise silicon oxynitride (SiON).
 4. The microphotoniclight source of claim 1, wherein said at least one Erbium-doped laserring comprises silicon oxynitride (SiON).
 5. The microphotonic lightsource of claim 1, wherein said at least one Erbium-doped laser ringcomprises silicon nitride Si₃N₄.
 6. The microphotonic light source ofclaim 1, wherein said optical pump comprises pump radiation of a 980 nmlaser source.
 7. The microphotonic light source of claim 1 furthercomprising a plurality of output planar waveguides that output eachlaser wavelength generated by said laser rings.
 8. The microphotoniclight source of claim 1 further comprising a plurality of output filterwaveguides.
 9. A method of forming a microphotonic light sourcecomprising: providing an optical pump; providing a plurality ofwaveguides that distribute optical pump power of said optical pump, saidwaveguides comprising silicon nitride (Si₃N₄) or silicon oxynitride(SiON) waveguides; and providing a plurality of Erbium-doped laser ringstructures that are coupled to at least one of said waveguides, each ofsaid Erbium-doped laser ring structures having radial dimensionsdesigned to match the resonance condition for both said optical pump andassociated wavelengths, said Erbium-doped laser ring structurescomprising Erbium-doped Si₃N₄ or SiON and having multi-wavelength signalemissions at approximately 1.55 μm; and providing a plurality of undopedrouting ring structures aligned along a principal waveguide bus todistribute the power from said optical pump to regions comprising saidErbium-doped laser ring structures; wherein said Erbium-doped laser ringstructures and said undoped routing ring structures are of varyingdimensions and act as selective pump routers and signal filters.
 10. Themethod of claim 9, wherein said waveguides comprise silicon nitrideSi₃N₄.
 11. The method of claim 9, wherein said waveguides comprisesilicon oxynitride (SiON).
 12. The method of claim 9, wherein said atleast one Erbium-doped laser ring comprises silicon oxynitride (SiON).13. The method of claim 9, wherein said at least one Erbium-doped laserring comprises silicon nitride Si₃N₄.
 14. The method of claim 9, whereinsaid optical pump comprises pump radiation of a 980 nm laser source. 15.The method of claim 9 further comprising providing a plurality of outputplanar waveguides that output each laser wavelength generated by saidlaser rings.
 16. The method of claim 9 further comprising providing aplurality of output filter waveguides.